The present invention relates to a light chopper for photometric instruments, and more particularly, the invention relates to improvements in motor driven diaphragms serving for periodically interrupting one or several beams of light.
Light choppers of the type outlined above are used for example in two beam photometers having two beams of radiation respectively traversing fluid filled reference and measuring chambers. The motor driven chopper interrupts the two beams to obtain an intensity modulation serving so to speak as a carrier signal which is amplitude modulated by absorption in the chambers.
The selection of the motor for driving such a chopper diaphragm is quite important. One usually prefers self-starting synchronous motors for various reasons. The diaphragm should sit directly on the motor shaft, without interpositioning of any transmission or coupling in order to obtain a quiet and steady run. The shaft should be short, so as to avoid additional bearing support outside of the motor. For the same reason one should use a rather flat motor casing.
The motor must exhibit sufficient torque particularly for starting, and particularly because just one motor driven diaphragm is to be used for both beams in such a two beam photometer. These beams are usually spaced apart by a considerable distance, so that the diameter of the chopper diaphragm is large accordingly. The torque of the motor increases of course with the diameter thereof.
The German printed patent application No. 2,132,973 describes a rather advantageously constructed light chopper, though not for two beam photometers, whose motor has a cylindrical rotor with a cover extending transversely to the direction of light as emitted by a source. The motor casing has a parallelly extending cover, and both covers have windows whereby the rotation of the rotor cover provides the light chopping function. See also the company brochure FA 55090b of March, 1963 (Motortype SSL) of AEG.
Generally speaking, two beam photometers can operate with in-phase or 180.degree.-out-of-phase chopping of the two beams. In either case the diaphragm is usually of winged construction or one uses a multi-aperture disk. It may be necessary in cases to change from one type of diaphragm to another one. If one uses the construction outlined in the preceding paragraph, the motor casing has to be opened and the rotating disk has to be removed and the new one with a different shutter pattern will be fastened to the shaft. One could have several motors available with different types of light chopping diaphragms, but that would require a large inventory.